1. Field of the Invention
The present invention relates generally to an optical information storage medium having lands and grooves both serving as recording tracks, and more particularly to an optical information storage medium which can obtain a stable push-pull signal during the scanning of a land/groove header region with a light beam.
2. Description of the Related Art
Optical disks are classified generally into read-only optical disks such as CD-ROMS, a write-once optical disks on which only writing is allowed, and rewritable optical disks such as magneto-optical disks and phase-change optical disks. Such optical disks have received attention as a memory medium that becomes a core in the recent rapid development of multimedia. A plurality of grooves are formed on a substrate of the optical disk in a concentric or spiral fashion to guide a laser beam to be directed onto the substrate. A flat portion defined between any adjacent of the grooves is called a land.
In a typical adjacent optical disk in the prior art, either the lands or the grooves are used as recording tracks on which information is recorded. Accordingly, a header portion composed of a plurality of phase pits preliminarily formed can be configured by a greatly simple method. However, a recent important technical subject to be considered is to increase recording density by using both the lands and the grooves as the recording tracks to thereby decrease the track pitch. In this respect, various methods for realizing this subject have already been proposed.
In a conventional optical disk adopting a land/groove recording method, the optical depth of each groove is set to about .lambda./8 (.lambda.: operating wavelength) in general, and the optical depth of each phase pit in the header portion is also set to about .lambda./8 in general. The reason for this setting is that in a magneto-optical recording medium, for example, if the optical depth of each groove is set larger than .lambda./8, the reproduction signal becomes too small, whereas if the optical depth of each groove is set smaller than .lambda./8, the sufficient quality of header signal itself cannot be obtained.
More specifically, the header portion consists of a land header portion for the lands as recording tracks and a groove header portion for the grooves as recording tracks. The land header portion is formed on an extension of each land in a space defined by once interrupting each groove, and the groove header portion is formed on an extension of each groove in this space. Alternatively, the land and groove header portions are formed at a mirror portion on an extension of the boundary between each groove and its neighboring land. The land and groove header portions are shifted from each other in the circumferential direction of an optical disk.
As another conventional land/groove recording method, a continuous groove is formed on the substrate. The groove header portion is formed by modulating the width of each groove, and the land header portion is formed with general phase pits. Each phase pit and each groove have the same optical depth. Also in this conventional method, the land header portion and the groove header portion are not adjacent to each other in the radial direction of an optical disk. That is, the land and groove header portions are shifted from each other in the circumferential direction of an optical disk.
In the conventional land/groove recording, the grooves are formed in a data region on the substrate in a concentric or spiral fashion, and the flat land is defined between any adjacent ones of the grooves. Each groove is interrupted once at the header region. Accordingly, the groove header portion for each groove as a recording track and the land header portion for each land as a recording track are located as phase pits in the Land/Groove header region where each groove is interrupted once. The optical depth of each phase pit is set to about .lambda./8 (.lambda.: operating wavelength), which is the same as the optical depth of each groove.
As a tracking error detecting method, a push-pull method and a heterodyne method, for example, are known. The push-pull method is a method utilizing the fact that the distribution of reflected light from an optical disk changes according to a positional relation between a beam spot of a laser beam focused on the optical disk by an objective lens and each groove formed on the optical disk, thereby effecting tracking error detection. When the center of the beam spot lies on the center line of each groove, the distribution of reflected light is uniform, whereas when the center of the beam spot is deviated from the center line of each groove, the distribution of reflected light become nonuniform, that is, it is shifted from the center line of each groove to the right or the left.
Accordingly, tracking error detection can be performed in the following manner. A reflected beam from an optical disk is made to enter a hologram diffraction grating for equally dividing the reflected beam into two beams along a line parallel to direction of information recording on the optical disk when the center of a beam spot directed on the optical disk lies on the center line of each groove. Then, the two beams obtained above are made to enter different photodetectors A and B. As a result, a tracking error signal TES can be expressed as follows: EQU TES=fa-fb
where fa and fb are the outputs from the photodetectors A and B, respectively.
Accordingly, tracking error detection can be performed according to a value of TES.
By setting the optical depth of each groove formed on the optical disk to .lambda./8 where .lambda. is the wavelength of a laser beam incident on the optical disk, the change in the distribution of reflected light due to variations in the focusing position of the laser beam is maximized. For this reason, the optical depth of each groove is set to .lambda./8 in the conventional method.
In the conventional method mentioned above, the optical depth of each phase pit formed as the groove header portion for each groove serving as a recording track is the same as the optical depth of each phase pit formed as the land header portion for each land serving as a recording track. Further, the optical depth of each groove is about .lambda./8, and the optical depth of each phase pit is .lambda./8 at the maximum.
A remarkably characteristic point in this structure is that as far as the optical depth of each groove formed on the substrate of the optical disk falls within .lambda./4, the polarity of a so-called push-pull signal (track error signal) is constant. Since each phase pit at the groove header portion and each phase pit at the land header portion have the same optical depth, the polarities of the push-pull signals with respect to both the phase pits are the same.
However, the groove header portion and the land header portion are radially shifted from each other by one track. Accordingly, when a laser beam spot scanning a certain groove track enters its Land/Groove header region, the beam spot successively scans the phase pits at the groove header portion. At this time, the polarity of a push-pull signal due to the phase pits at the groove header portion is the same as the polarity of a push-pull signal during scanning of the groove track, so that tracking servo is stably operated.
After passing the groove header portion, the beam spot scans a flat region interposed between two adjacent lines of the phase pits at the radially adjacent land header portions. Each phase pit of these land header portions defining the flat region therebetween has an optical depth of .lambda./8 equal to that of each groove. Accordingly, the polarity of a push-pull signal due to the phase pits at each land header portion is inverted from the polarity of a push-pull signal due to the phase pits at the groove header portion.
That is, there occurs a rapid inversion of the polarity of a push-pull signal at the boundary between the groove header portion and the successive land header portion, causing a problem in that the flat region between the adjacent land header portions cannot be scanned in this case. To solve this problem, it is necessary to provide any means for detecting a timing corresponding to the above boundary and electrically inverting the polarity of a push-pull signal at the land header portion. As a result, an optical disk drive in the prior art becomes complicated in configuration.